Physics for Geologists, Second edition

Gravity 41
Figure 3.9 The siphon at work. When one container is raised, it acquires poten-
tial energy due to the difference in elevation, h, and this potential
energy is realized by moving the water from the higher to the lower
container.
larger pressure in the longer arm of the tube. Clearly there is no pressure
discontinuity inside the tube, and we can assume that the smallest pressure in
the tube is sensibly at the crest. The pressure inside the tube at the water levels
of the containers is the same on both sides - the source and the destination
both being at atmospheric pressure at the surfaces of the water.
The siphon can be explained in terms of the weight of fluid in the two
arms of the tube, where clearly the longer arm has a greater weight of water
than the shorter. The heavier side sucks the fluid from the lighter. Again,
this has a limit due to the 'vacuum' created if the tube is sufficiently high.
This is a valid argument for liquids, but it still comes down to energy not
pressure. You will remember that the dimensions of energy are a force or
weight times a height above an arbitrary horizontal datum, such as the table
top. So the energy of the water in the higher container is greater than that
of the water in the lower container, and the extra weight in the longer tube
sucks the water from the shorter tube.
The same principles apply to a chain passing from one bucket to another
over a sheave or pulley. If you lift one bucket, the chain starts moving into
the lower bucket. Energy and weight are involved, but not pressure. It does
not matter how much chain is in the buckets: it is the relative energy in the
chain on either side of the sheave or pulley that matters.
To move anything requires energy and work.
Copyright 2002 by Richard E. Chapman